1. Technical Field
The present invention relates to a heating apparatus using electromagnetic induction and a fixing apparatus for fixing a toner image onto a recording material using the heating apparatus.
2. Related Art
Generally, in an image forming apparatus using powder toner, in a process to fix a toner image, a method of electrostatically transferring a toner image onto a recording medium, then placing the recording medium between a heating member and a pressure member, and heat-melting the toner image thus press-fixing the toner image to the recording medium, is widely employed. For the heating of the heating member, an arrangement where the heating member has a conductive layer so as to generate heat by the conductive layer by electromagnetic induction heating has been proposed. The electromagnetic induction heating provides an exciting coil to generate a varying magnetic field near the conductive layer (heating member) and causing the conductive layer to generate heat by an eddy current generated in the conductive layer. According to the electromagnetic induction heating, as the heating member is directly heated and the range of high temperature by heating is extremely limited, the heating member can be heated to a predetermined temperature in a short time. Accordingly, in comparison with heating using a halogen lamp or the like as a heating source, warm-up time of the fixing apparatus can be reduced, and electric consumption can be reduced.
On the other hand, as the heating member (fixing member), as well as a heating roller, an endless fixing belt is generally used. The endless fixing belt is a belt put around plural support rollers, or is a belt with an inside pressure member and is circulate-driven without a roller. The fixing belt has a thin heat-resisting resin layer or the like as a base layer. As the thermal capacity of the fixing belt is smaller than that of the heating roller, the warm-up time is shorter in comparison with that of the apparatus using the heating roller. Further, in the non-expanded type fixing belt, the area to be contact with another member can be reduced, thereby heat transfer to the other member can be reduced. Accordingly, further efficient warming up can be performed.
In a fixing apparatus where an endless belt as a heating member is heated by electromagnetic induction, when the endless belt is put around plural rollers, the exciting coil is provided to face the inner surface or outer surface of the belt. On the other hand, when the endless belt is circulate-driven without a roller, the exciting coil is provided in a position close and opposite to the outer peripheral surface of the endless belt. Then, a varying magnetic field is generated in a direction through the endless belt, and an eddy current is induced around the magnetic field.
Generally, a high frequency current supplied to the exciting coil is generated by switching a direct current at a high frequency, and constant current control or constant energy control is performed. Further, upon electric power supply to the exciting coil, the temperature of the fixing member as a heated body is detected with a temperature sensor and the amount of supplied electric power is controlled and/or power supply ON/OFF control is performed so as to maintain a predetermined temperature.
The heating apparatus using the electromagnetic induction and the fixing apparatus using the heating apparatus have advantages as described above while they also have disadvantages. One of the problems is that as the speed of temperature rising is fast, a safety measure against abnormal high temperature (overheated) state of the heated body (e.g., the heating member) cannot be taken without difficulty. As the safety measure against abnormal high temperature, in the case of the fixing apparatus using the conventional halogen lamp, a thermostat, a temperature fuse or the like is provided in contact with or in the vicinity of a fixing roller, such that when the temperature of the fixing roller becomes a predetermined temperature, a current path to the halogen lamp is blocked and overheating of the fixing roller is prevented.
However, the thermostat or the temperature fuse operate with a certain degree of time delay. That is, the detection of the actual temperature of the heated body is delayed, and when the thermostat or the like detects a predetermined reference temperature and operates, the temperature of the heated body is higher than the reference temperature. In the case of the halogen lamp where the speed of temperature rising is slow, the above overheating of the fixing roller can be sufficiently prevented. In the case of the electromagnetic induction heating, however, as the speed of temperature rising is fast, the heating cannot be appropriately controlled. Especially when a belt having a small thermal capacity is used as the heated body, the above problem is particularly significant.
FIG. 9 is a graph schematically showing an example of the difference among the temperature of the heated body, that of a bimetal and that of a fuse to detect the temperature of the heated body.
In the case of the thermostat where the bimetal is directly in contact with the heated body, it takes about 50 to 60 seconds (t0 to t1) to detect abnormal high temperature T0 of the heated body. Further, in the case of the temperature fuse, as the temperature fuse cannot be brought into direct contact with the fixing roller, it takes about 100 seconds (t0 to t2) to detect the abnormal high temperature while the temperature of the fixing roller increases from the abnormal temperature T0 to temperature T1 or T2. In a case where a rotating body such as a belt having a small thermal capacity is used, the inclination of temperature rising is steeper, the temperature further increased (T1, T2) during the delay of thermal transmission is higher. Accordingly, it is necessary to use a high heat-resistant member to resist such high temperature. On the other hand, when the reference temperature for control is set to a temperature (T3 or T4) lower than the abnormal temperature T0 of the heated body, the error of temperature detection is increased. The tendency is significant as the inclination of temperature rising is steeper.